In extreme cases, removal of the floorboards, replacement and refinishing might be your only option for the darkest water stained areas.
You've decided to fix that dark water spot on your hardwood floor that's been hidden under a throw rug for years. Most dark water stains on hardwood floors are the result of spilled liquids seeping into the polyurethane layers and the uppermost layers of the wood flooring. Removing these stains can restore the beautiful look of your hardwood floors. Restoring your floor will require some elbow grease and patience. The steps listed below work in succession from least invasive (less work) to the most extreme treatment of stained floors. Remember that darkened stains usually indicate the water has seeped below the polyurethane finish to the wood below.
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Wipe away any surface dirt and grime with a damp cloth. Working on a dirty floor can result in scratches and gouges, marring the finish even more. Lightly rub the area with the No. 1 steel wool to scour the area, blending with lighter rubbing into the unstained areas of the floor. This feathering will help with future staining and refinishing.
Soak a clean rag in vinegar and place the rag onto the stain. Allow the vinegar to work into the stain for 5 to 10 minutes. You may need to work in sections for larger stains and apply the vinegar multiple times to see stain lightening.
Test an application of hydrogen peroxide in an inconspicuous area of the floor. Hydrogen peroxide will lighten the stain with successive applications. Soak a cloth and apply to the dark water stain and allow 5 to 10 minutes for it to work. You can dilute the peroxide with water at first, working up to a stronger solution, if needed.
Apply a layer of mineral spirits to the dark water stained area using the steel wool to lightly scour the floorboards. Wipe away any excess. In this case, you're beginning to remove the protective polyurethane layers. Work the mineral spirits into the wood. You can also substitute hardwood floor cleaner at this stage to help remove the water stain from the wood.
Mix a solution of four parts water to one part bleach and spray or wipe onto the exposed wood surface. Bleach will lighten the stain with successive applications. If you aren't seeing appropriate results, strengthen the solution and reapply. Sometimes, sanding the surface with fine sandpaper will help release the layers of polyurethane.
Protect the area with a layer of hardwood floor wax appropriate for the type of flooring after the stain has been removed. Scouring the wood flooring surface with steel wood or sandpaper removes the protective wax finish.